Enable loop ctrl with placeholder for SSA and backward ctrl move

[tancheng]

Enable loop ctrl with placeholder for SSA and backward ctrl move, basically,

• neura.reserve is added as a phi's operand if the real dependent value is defined within the same block but after the phi's creation
• Insert the backward ctrl move after the real value's define, to indicate there is a loop-carried dependency without violating SSA

The provided test can demonstrate this PR's funtionality:

func.func @loop_test() -> f32 {
  %n = llvm.mlir.constant(10 : i64) : i64
  %c0 = llvm.mlir.constant(0 : i64) : i64
  %c1 = llvm.mlir.constant(1 : i64) : i64
  %c1f = llvm.mlir.constant(3.0 : f32) : f32
  %acc_init = llvm.mlir.constant(0.0 : f32) : f32

  llvm.br ^bb1(%c0, %acc_init : i64, f32)

^bb1(%i: i64, %acc: f32):  // loop body + check + increment
  %next_acc = llvm.fadd %acc, %c1f : f32
  %i_next = llvm.add %i, %c1 : i64
  %cmp = llvm.icmp "slt" %i_next, %n : i64
  llvm.cond_br %cmp, ^bb1(%i_next, %next_acc : i64, f32), ^exit(%next_acc : f32)

^exit(%result: f32):
  return %result : f32
}

After transformation:

func.func @loop_test() -> f32 attributes {accelerator = "neura"} {
  %0 = "neura.constant"() <{predicate = true, value = 10 : i64}> : () -> !neura.data<i64, i1>
  %1 = "neura.constant"() <{predicate = true, value = 0 : i64}> : () -> !neura.data<i64, i1>
  %2 = "neura.constant"() <{predicate = true, value = 1 : i64}> : () -> !neura.data<i64, i1>
  %3 = "neura.constant"() <{predicate = true, value = 3.000000e+00 : f32}> : () -> !neura.data<f32, i1>
  %4 = "neura.constant"() <{predicate = true, value = 0.000000e+00 : f32}> : () -> !neura.data<f32, i1>
  %5 = neura.reserve : !neura.data<i64, i1>
  %6 = "neura.phi"(%1, %5) : (!neura.data<i64, i1>, !neura.data<i64, i1>) -> !neura.data<i64, i1>
  %7 = neura.reserve : !neura.data<f32, i1>
  %8 = "neura.phi"(%4, %7) : (!neura.data<f32, i1>, !neura.data<f32, i1>) -> !neura.data<f32, i1>
  %9 = "neura.fadd"(%8, %3) : (!neura.data<f32, i1>, !neura.data<f32, i1>) -> !neura.data<f32, i1>
  neura.ctrl_mov %9 -> %7 : !neura.data<f32, i1> !neura.data<f32, i1>
  %10 = "neura.add"(%6, %2) : (!neura.data<i64, i1>, !neura.data<i64, i1>) -> !neura.data<i64, i1>
  neura.ctrl_mov %10 -> %5 : !neura.data<i64, i1> !neura.data<i64, i1>
  %11 = "neura.icmp"(%10, %0) <{cmpType = "slt"}> : (!neura.data<i64, i1>, !neura.data<i64, i1>) -> !neura.data<i1, i1>
  "neura.return"(%9) : (!neura.data<f32, i1>) -> ()
}

[ShangkunLi]

LGTM

[tancheng]

LGTM

lol, i found the ctrl_mov is inserted wrongly (when I implementing recMII util func in a new branch), will fix forward ASAP :-)